Evolution of Kelvin-Helmholtz Instability in the Fan-spine Topology

We use multiwavelength imaging observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory to study the evolution of the Kelvin-Helmholtz (K-H) instability in a fan-spine magnetic field configuration. This magnetic topology exists near an active region AR12297 and is rooted in a nearby sunspot. In this magnetic configuration, two layers of cool plasma flow in parallel and interact with each other inside an elongated spine. The slower plasma flow (5 km s-1) is the reflected stream along the spine's field lines from the top, which interacts with the impulsive plasma upflows (114-144 km s-1) from below. This process generates a shear motion and subsequent evolution of the K-H instability. The amplitude and characteristic wavelength of the K-H unstable vortices increase, satisfying the criterion of the fastest-growing mode of this instability. We also describe how the velocity difference between two layers and the velocity of K-H unstable vortices are greater than the Alfvén speed in the second denser layer, which also satisfies the criterion of the growth of the K-H instability. In the presence of the magnetic field and sheared counterstreaming plasma as observed in the fan-spine topology, we estimate the parametric constant ? = 1, which confirms the dominance of velocity shear and the evolution of the linear phase of the K-H instability. This observation indicates that in the presence of complex magnetic field structuring and flows, the fan-spine configuration may evolve into rapid heating, while the connectivity changes due to the fragmentation via the K-H instability.